As is well known in aircraft and gas turbine engine technology, the constant flights of the aircraft subject the aircraft components and particularly the components of the engine to severe vibrations. One of the problems encountered with the hollow airfoils of the stator vanes of the gas turbine engine is the propensity for premature cracking of the walls of the airfoil. Experience has shown that subjecting these vanes to severe vibrations after time the initiation of the crack will generally occur at the leading edge of the airfoil on its smooth side where there are no air cooling holes. The cracking typically initiates at the high stress field determined by the local stress concentration factor and the high bending stress. One approach that has been tried for solving this cracking problem is to "soften" the local "hard" point between the thin vane surface and the stiff leading and trailing edges. This technique has not proven to be satisfactory.
Another technique and one that is more pertinent to the present invention is disclosed in U.S. Pat. No. 2,689,107 which is commonly assigned to the assignee of this patent application. This patent discloses a frictional damper that utilizes a sinusoidal shaped spring member extending longitudinally in the cavity of a hollow vane or blade that extends from the tip to the root of the blade. The tip cap is removed and rewelded into place to accommodate insertion and removal of the spring. The lands of the spring bear against the opposite surfaces of the pressure and suction walls where the frictional contact effectuates the damping and absorbs the energy generated by the vibratory motion. This design requires that the end cap which is located at the thinnest section of the blade to be removed so as to insert the spring in the blade and that the spring extend the expanse from the leading to trailing edges in the cavity. This is true whether the design utilizes a single spring or multiple springs configurations. While this damper has proven to be efficacious, it is limited to being incorporated in the original design and fabricated at the initial fabrication or production of the blade. Further, the prior art configuration requires that the blade be of the type that includes an end or tip cap. Unlike the present invention, the prior art design isn't applicable for blades that are existing and are in service where field repair is necessary. Hence, the structure disclosed in the U.S. Pat. No. 2,689,107, does not lend itself to be fabricated as a retrofit item as is the case of the present invention.
This invention contemplates utilizing a judiciously designed spring damper inserted in the hollow airfoil along the longitudinal direction so that the elements of the spring bear against the inside surface of the pressure side and suction side to absorb the energy generated by these vibrations. The damper consists of a single elongated member bent along its longitudinal axis forming a U-shaped or V-shaped spring in cross section. The outer surfaces of the arms of the U-shaped or V-shaped spring bear against the inner surfaces of the opposing airfoil walls and by virtue of the frictional action between the spring and the wall the vibratory energy is dissipated. The invention as described hereinbelow is adapted for installation in existing stator vanes and lends itself to being fabricated as a retrofit sub-assembly.
The spring damper of this invention in one embodiment is easily installed through a small hole formed on the one end of the airfoil and held in position by a rivet or suitable bond. The spring damper of this embodiment is particularly suited for use in retrofitting existing airfoils. In another embodiment the spring damper carries a tang formed at one end of the spring damper that fits into a hole formed at one end of the airfoil and is riveted or suitably bonded thereto. The inner surface of the opposing walls (suction side and pressure side) include a projection extending into the hollow of the airfoil for defining a stop that serves to guide and locate the damper as it is being installed and once it assumes its proper position.
In either instance the spring damper is securely held at one end while the other end is free to move as in a cantilever mount. In addition to being relatively easy to manufacture and install and capable of installation in existing airfoils, (although it can also be utilized in original designed hardware) this invention does not require separate end caps as is the case of the heretofore known designs. In addition, and not by way of limitation, this invention will function in airfoil designs which contain a large amount of taper, twist, and a nonuniform cavity. As it is easy to install, it is likewise easy to remove which is very important in aircraft applications where continuous and repetitive inspections of the damper and airfoil are required and is lighter and less expensive than heretofore known designs.